This invention relates to manufacturing of sodium hydroxide. More particularly, the invention relates to manufacturing of sodium hydroxide and ammonium sulfate by electrolyzing sodium sulfate.
Demonstrated worldwide demand for some sodium-based chemicals, particularly for sodium hydroxide (caustic soda), has been on the rise in recent years. This strong demand, which is forecast to continue, keeps this chemical in tight supply position, thereby holding the price at a high level. This trend is not the same with respect to all sodium-based chemicals. In particular, the demand for sodium sulfate and, as a consequence, the price of this chemical is declining at the same time as the demand for caustic soda is rising.
This declining trend in the demand for and prices of sodium sulfate combined with the strong demand for and relatively high prices of other sodium-based chemicals, in particular of caustic soda, created a need for a simple and economical process for producing sodium hydroxide from sodium sulfate as feedstock. This need is even more strongly perceived in countries endowed with vast natural resources of sodium sulfate. This is, for example, the case in Canada, which has large deposits of natural sodium sulfate located in Southern Saskatchewan.
The most direct process for producing sodium hydroxide from sodium sulfate is the electrolytic conversion of an aqueous solution of sodium sulfate into aqueous solutions of sulfuric acid and caustic soda. Numerous implementations of this process are known in the prior art. Most of them make use of electrolytic cells employing diaphragms or ion permeable membranes to separate the product solutions from the feed solution, thus avoiding contamination of the products by the feedstock material.
U.S. Pat. No. 2,829,095 discloses a process for the production of acidic and alkaline solutions by electrolysis of a salt solution in a multi-compartment electrolytic cell partitioned by a plurality of anion and cation exchange membranes. The patent also discloses the use of the process for direct production of sodium hydroxide and sulfuric acid from Glauber's salt (sodium sulfate decahydrate).
U.S. Pat. Nos. 3,135,673 and 3,222,267 claim a method and apparatus for converting aqueous electrolytic salt solutions to their corresponding acid and base solutions. A three or four compartment electrolytic cell separated by a cation exchange membrane and one or two porous, non-selective diaphragms is used for this purpose. When a solution of sodium sulfate is used as the salt solution, solutions of sodium hydroxide and sulfuric acid or sodium bisulfate are produced.
U.S. Pat. No. 3,398,069 claims a process for the electrolysis of an aqueous saline electrolyte in a multicellular device having cells separated by gas permeable electrodes and further partitioned by microporous fluid-permeable diaphragms or ion-permselective membranes. When applied to a solution of sodium sulfate, the process produces solutions of sodium hydroxide and sulfuric acid.
U.S. Pat. No. 3,907,654 discloses an electrolytic cell particularly useful in electrolysis of sodium sulfate to form sulfuric acid and sodium hydroxide. The cell, which does not employ any ion permeable membranes, comprises a housing having a parent solution chamber and two electrode compartments located on the lower side of the housing and separated from each other but in communication with the parent solution chamber and positioned vertically beneath or above. Mounted within the electrode compartments are an anode and a cathode, each of which is porous to permit passage of a product solution therethrough. The product solutions of sodium hydroxide and sulfuric acid separated by gravity forces are withdrawn through the porous electrodes.
U.S. Pat. No. 4,561,945 claims a process for producing sulfuric acid and caustic soda by electrolysis of an alkali metal sulfate in a three compartment membrane cell having a hydrogen depolarized anode. Hydrogen gas in the anode chamber is oxidized to produce hydrogen cations which migrate to the central (buffer) chamber through a membrane and combine with the sulfate anions from the alkali metal sulfate solution to produce sulfuric acid. Alkali metal ions are transported across another membrane to the cathode chamber to produce caustic and gaseous hydrogen. Both membranes used in the cell are cation selective membranes.
A similar process for increasing concentration of sulfuric acid in solutions containing an alkali metal sulfate, sulfuric acid and alkaline earth metal ions is disclosed in U.S. Pat. No. 4,613,416. Also in this case the anode compartment and the cathode compartment of a three compartment cell are each bounded by cation exchange membranes.